In a conventional cathode ray tube (CRT), an electron gun comprised of a cathode and a plurality of aligned charged grids generates and forms energetic electrons into a beam and focuses the electron beam on the inner surface of a phosphor-coated faceplate. The electron gun is comprised generally of a beam forming region (BFR) and a beam focusing region. A focus voltage V.sub.F and an accelerating voltage V.sub.A are applied to various grids in the focusing portion of the electron gun, where V.sub.A&gt;V.sub.F. The Einzel-type electron gun is a well known electron gun design which has been used for many years in CRTs. An advantage of the Einzel-type electron gun is that only one anode voltage V.sub.A source is required.
Referring to FIG. 1, there is shown a longitudinal sectional view of a prior art Einzel lens electron gun 10 for generating, accelerating and focusing an electron beam 12 on a CRT's faceplate (not shown for simplicity). Electron gun 10 includes a heated cathode 14 for generating energetic electrons and a plurality of charged grids aligned along axis A-A'. Electron gun 10 further includes a G1 control grid 16, a G2 screen grid 18, a G3 grid 20, a G4 grid 22, and a G5 grid 24. The combination of the G1 control grid 16, the G2 screen grid 18, and the facing portion of the G3 grid 20 comprise the BFR in electron gun 10. The G3 grid 20, the G4 grid 22 and the G5 grid 24 form the Einzel lens, or main lens, of the electron gun for focusing electron beam 12. The G3 grid 20 and the G5 grid 24 are coupled to an anode voltage (V.sub.A) source 21, while the G4 grid is coupled to a focus voltage (V.sub.F) source 19.
Because the G4 grid 22 is maintained at a much lower voltage than that of the G3 and G5 grids 20, 24 and because the velocity of the electrons in beam 12 is proportional to the square root of the accelerating voltage, or EQU v=k.times.V, [Eq. 1]
where
v=velocity of electrons, PA1 V=accelerating voltage, and PA1 k=proportional constant,
the velocity of the electrons along axis A-A' in the vicinity of the G4 grid will be much less than that adjacent the G3 and G5 grids. In effect, the electrons slow down as they transit the G4 grid 22.
The electrons because of their lower velocity in this portion of the Einzel lens are more subject to stray electrostatic fields within the Einzel lens. Stray electrostatic fields arise from stray space charge effects due to electron deposit on an electrode support rod, or glass bead, (described below) as well as on the inner surface of the neck portion 32a of the CRT's glass envelope 32. The conventional low voltage Einzel lens design shown in FIG. 1 provides for overlapping of the G3 and G4 grids 20, 22 and the G4 and G5 grids 22, 24 to limit stray electrostatic fields introduced into the electron gun 10. While overlapping adjacent grids reduces the stray electrostatic field within the electron gun, the difference in diameters of the adjacent grids which permits this overlapping arrangement renders it more difficult to assemble the electron gun as described in the following paragraphs.
Referring to FIG. 2, there is shown a sectional view of a CRT 30 incorporating the electron gun 10 of FIG. 1, where the electron gun is shown in a side elevation view. CRT 30 includes a glass envelope 32 comprised of an elongated, narrow neck portion 32a, an expanding funnel portion 32b, and a glass faceplate 32c securely attached in a sealed manner to the CRT's funnel portion. An end of the CRT's neck portion 32a is fitted with a base member 34 typically comprised of plastic for attaching a plurality of conductive pins 36 to the end of the CRT envelope 32. Pins 36 extend through an end of the CRT's neck portion 32a and are electrically coupled to the various grids described above by means of a plurality of conductors 38. Pins 36 are further coupled to a power supply 52 for providing V.sub.A, V.sub.F and other electrical signals to the various components within CRT 30. For simplicity, FIG. 2 shows the V.sub.F, V.sub.A and other electrical signal sources as a single power supply 52. Power supply 52 is also coupled via an anode button 44 extending through the CRT's funnel portion 32b to a conductive coating 46 disposed on the inner surface of the CRT's glass envelope 32. The high anode voltage V.sub.A is provided to the CRT's screen via the anode button 44 and conductive coating 46. A conductive convergence cage 54 is disposed within the CRT 30 and is maintained in position therein by means of a snubber spring 48 which is disposed about the convergence cage and engages conductive coating 46. A video signal source (not shown for simplicity) provides video information to either the cathode or to the G1 control grid 16 for presenting a video image on the CRT's faceplate 32c. The inner surface of faceplate 32c is provided with a layer of phosphor elements 50, each of which illuminates when the electron beam 12 is incident thereon.
Convergence cage 54 is maintained at the anode voltage V.sub.A and is typically coupled to the high end of the G5 grid 24. The G1 control, G2 screen, G3, G4 and G5 grids 16, 18, 20, 22 and 24 are each provided with two or more metallic tabs, or studs, for attaching each of the grids to two or more insulating electrode support rods which are shown as elements 40 and 42 in FIG. 2. As shown for the case of the G2 screen grid 18, first and second metallic tabs 28a and 28b extend from the grid and are respectively attached to the first and second electrode support rods 40 and 42. The first and second electrode support rods 40, 42 as well as the convergence cage 54 provide support for electron gun 10 within the neck portion 32a of the CRT's glass envelope 32.
In assembling electron gun 10, a grid positioning/alignment mechanism, shown in FIG. 1 in simplified schematic and dotted-line form as element 26, is used to align the G3, G4 and G5 grids 20, 22 and 24 forming the Einzel lens. The grid positioning/alignment mechanism 26 engages respective outer portions of the G3, G4 and G5 grids 20, 22 and 24 for mutually aligning these three grids as well as for aligning these grids with the G1 control and G2 screen grids 16, 18 during attachment to the first and second electrode support rods 40, 42. To maintain a small electron beam spot size and to ensure proper focusing of electron beam 12 on faceplate 32c, it is essential that the various charged grids be concentrically aligned with respect to axis A-A'. Employment of the grid positioning/alignment mechanism 26 shown in FIG. 1 for aligning the grids makes it impossible to use mandrel beading which is a common technique used in CRT assembly to control the concentricity of the stack of electrodes along the electron beam path. The concentric alignment of the overlapping G3, G4 and G5 Einzel lens grids 20, 22 and 24 when employing a conventional grid positioning/alignment mechanism 26 is controlled by the outer circumference of these grids. The accuracy of the concentric positioning of these grids along axis A-A' is limited by the mechanical tolerance of the various individual components. These mechanical tolerances, such as grid thickness and out-of-roundness, render it virtually impossible to precisely align the grids along a common axis.
The present invention addresses the aforementioned limitations of the prior art by providing an electron gun having an Einzel lens which permits the use of mandrel beading for electron gun alignment and assembly while providing a high degree of shielding against stray electrostatic fields within the electron gun.